Fundamentals of Physics Extended (10th Edition)

by
Halliday, David; Resnick, Robert; Walker, Jearl

Published by
Wiley

ISBN 10:
1-11823-072-8

ISBN 13:
978-1-11823-072-5

Chapter 4 - Motion in Two and Three Dimensions - Problems: 70d

Answer

Negative x direction

Work Step by Step

The boat is moving upstream in the positive x direction at 14km/h and the water is flowing at 9km/h. Since the boat is moving upstream, it is traveling AGAINST the current, meaning that the river is flowing in the negative x direction.
Therefore, to find the speed of the boat relative to the ground we need to subtract the two (opposite directions) which would be:
14km/h (speed of boat) - 9km/h (speed of water) = 5km/h in the positive x
The boy moves 6km/h from the front to the back of the boat (negative x direction) relative to the boat. Therefore he moves 5km/h (boat speed) - 6km/h (child speed) = -1km/h relative to the ground